A. Field of the Invention
The present invention relates to HID light sources with relatively high power consumption. In particular, it relates to controlling the amount of power provided to an HID light source in order to adjust the quantity of light output and the amount of energy consumed.
B. Issues in the Present State of the Art
The above-mentioned HID light sources are relatively high power and consume considerable amounts of energy per hour. Energy conservation is important because many lighting systems, especially sports lighting systems, utilize a plurality of light sources (e.g. a plurality of poles each with a plurality of light fixtures or luminaries—see FIG. 1 for one pole of plural fixtures) that operate for many hours each year. One common method to conserve energy is to operate the lighting system at a lower light output during times when less illumination is deemed acceptable by the owners, participants, or by standards of play set forth by lighting organizations.
One such lighting organization in the United States is IESNA (Illuminating Engineering Society of North America). Using sports lighting as an example, IESNA Publication RP-6-01 provides minimum recommended illumination levels based on the type of sport, the players' skill level, and/or the number of spectators. However, many lighting systems are used for multiple purposes which may have different lighting needs, e.g. a soccer field that is used for practice but also used for tournaments with spectators. A lighting system like this application would need to be designed for the highest level of illumination needed to allow for tournament play based on the skill level of the players and the number of spectators. However, this higher level of illumination is generally only needed for tournament play, which is usually considerably less overall time than for practice. Lighting for practice could be operated at a lower level of illumination per the IESNA guidelines, thus saving energy.
One method of adjusting the amount of light provided to a target with varying lighting needs, without dimming the lights, is to have switching circuits that only turn on a subset of the total set of lights or luminaries of the whole system for lower levels, and the entire set of the lights or luminaries of the whole system on for high levels. While this method is more efficient in regards to energy ratio to light output, additional lights are often required to ensure adequate beam distribution over the target area for all switching levels. This can add cost and energy usage to the system. Also, the lamps in different switching groups may accumulate uneven operating hours if some groups are used more frequently than others. This imbalance of operating hours can cause light level uniformity issues for some systems due to uneven lamp depreciation as well as different maintenance needs. In addition, switching mechanisms are required to turn on the appropriate lights for each illumination level which adds cost and complexity.
Methods do exist to control the amount of power provided to each lamp to reduce light output from the lights, but they generally require installation of additional circuitry components. Adjusting power to a lamp has a direct impact on the quantity of light output from the lamp. For each percent of power change, the light output percentage changes by approximately 1.5 percent. This relationship between power and HID light output is well known in the field of lighting.
The most common method of adjusting the power to a lamp for the purpose of reducing light output (sometimes referred to as dimming the lamp) is to change the amount of capacitance in the system that is related to the lamp. Capacitors in a HID lighting circuits restrict the amount of current the lamp is able to draw. Since the arc tube of an HID lamp is non-resistive, it will continue to draw power until it self-destructs if it is not regulated by a capacitor.
One known way to adjust capacitance for dimming purposes is to connect together multiple capacitors in parallel and control them by means of contactors or other methods of switching. At initial start up, the lamp is generally operated at full power for a period of time by switching in a commensurate cumulative capacitance from a plurality of capacitors to allow the lamp to stabilize. Then capacitance is removed from the circuit by opening the contacts on the contactor to switch out at least one capacitor, which results in significant less power to the lamp, thus both dimming the lamp and conserving energy. An example of this type of system is disclosed in U.S. Pat. No. 4,994,718 (incorporated by reference herein) (see also the MULTI-WATT™ product commercially available from Musco Lighting, Oskaloosa, Iowa 52577 USA (“Musco”)). This method of starting in high level, power, or mode and dropping to a lower mode is many times used because the wattage levels for dimming are below the threshold at which the lamp is able start up and operate without first operating at near full wattage for the initial warm-up period of 15-20 minutes. For example, for a 1500 watt (“W”) metal halide lamp, ANSI C78.43-2005 (“American National Standard for electric lamps—Single-Ended Metal Halide Lamps”) specifies the lower lamp starting wattage threshold to be 1200 W. Testing of lamps utilized in Musco's sports lighting systems have indicated the ability to start and operate at slightly lower levels without any material impact to the lamp characteristics.
Using multiple capacitors to regulate the power to the lamp is somewhat limited due to practical matters. Additional space is needed for the additional components and associated equipment. Extra switching components are needed to control them. There must be a plurality of capacitors for each lamp. If there are a number of lamps per pole, the number of additional capacitors that must be installed and wired into the control circuitry and enclosure box 8 (see FIG. 1) for that pole are multiplied by that number of lamps. For this reason, most capacitor systems used for dimming are limited to one step-down in wattage. This may not be sufficient for some purposes.
Capacitors are also used to regulate the power to the lamp to hold the light output at a generally constant level. One method of capacitors used in this manner is disclosed in U.S. Published Patent Application 2005/0184681 A1 (incorporated by reference herein) (see also the SMART LAMP™ product commercially available from Musco). While this is an efficient method of controlling the power to the lamp, there is room for improvement in this area. For example, it would be advantageous to be able to expand the power range.